"U not my dad" is used as a reply when someone who isn't a family member (specifically your dad) tells you to do something and you reply with, "you not my dad".
Step dad: "austin pick that shit up"
Austin : "u not my dad u always tryna say something fuckn stupid ass noodle head"
Guy 1: "Yo momma's so stupid, she sat on the TV and watched the couch."
Guy 2: "U not funny."
Guy 1: *Head explodes from the awesomness of the comeback.